We are using new methods for the noninvasive stimulation of the human cortex. Stimulation can be with a high~voltage electrical pulse or with magnetic stimulation. One purpose is to use these methods for noninvasive localization of different parts of the human cortex including motor cortex, sensory cortex and language cortex. Another purpose is to study cortical physiology in different disease states. We have continued to make advances in understanding the technical aspects of magnetic stimulation, defining the optimal method to map different body part representations in the motor cortex. We have compared the site of stimulation with the area of the brain activated in positron emission tomography (PET) scans by mapping the results of both studies onto the same MRI scan; correlation is excellent. Extensive effort has been devoted to the study of the inhibitory effects of brain stimulation. Silent periods are much longer with magnetic stimulation than with electrical stimulation suggesting that most of the effect comes from within the motor cortex. Topographic maps of the contralateral silent period of a muscle are generally lateral to those of the motor evoked potential of the same muscle. Mapping the ipsilateral muscles (in the hand) shows that for the same muscle the map of the ipsilateral muscle also falls lateral to the that of the contralateral muscle. Following~up our previous studies that showed reorganization of motor cortex pathways following anesthetic block of the forearm and hand, we have shown that this effect is probably mediated largely at the cortical level. Magnetic stimulation is able to detect enlargement of motor representations of the fingers associated with the acquisition of a digital skill in normal individuals and with the maintainance of the skill of Braille reading in the blind. These studies suggest that there is flexible allocation of cortical resources which is determined, in part, by behavioral factors. We have found that following exercise to the point of fatigue, the compound action potential of a muscle to peripheral nerve stimulation remains unchanged; however, the response in the same muscle to magnetic stimulation declines in amplitude. This shows that postexercise fatigue arises in the central nervous system.